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However, the Interactive Diagram had trouble pressing its advantage in the endgame, continually checking the King with its Queen instead of bringing in other pieces to support the Queen.

The Interactive Diagram is rather poor in end-games. I am aware of that, but did not consider it a high priority to improve. The purpose of the Diagram is to give the user a good impression of what a given chess variant is like, not to score as high as possible in a tournament. An end-game is often very different in character than the game it results from. For getting an idea of how to win with a specific piece the Checkmating Applets are a convenient tool.

The most useful definition of end-game that I know is when it is no longer fatal to involve your royal piece in the battle, and let it roam the board. Because royal pieces tend to have very limited power of motion, this requires planning many moves ahead, and the key positions where progress can be made in terms of gaining material can usually be reached through many different paths. To find those requires lare search depths, which can only be reached with the aid of a Transposition Table. With the aid of that you can avoid having to evaluate the same position over and over again depending on the path you used to reach it. The AI of the Interactive Diagram does not have a transposition table (yet). So the search depth can not be made large enough to allow it to see the ways to make progress, which makes it clueless. Middle-game tactics, on the other hand, is quite shallow in comparison, and transpositions are not very abundant, so not having a TT is not much of a drawback.

Chasing the royal piece with checks is a problem that can be prevented even without a TT, though. The trick is that it is in general useful to check. (It limits the number of moves available to the opponent, and who knows what opportunity will present itself after that?) But repetitive checking with the same piece in general leads to nothing, except when the piece has mating power on its own (such as the Lion, KNAD). So one can (lightly) penalize branches that contain multiple checks with the same piece in a row. While checking with two different pieces usually leads to a quick checkmate.

Problem is that currently the Interactive Diagram is not even aware when it is in check, except in Quiescence Search (where it then suppresses scoring the position with the current material balance, and forces search of all possible check evasions), or when it could not find any legal move. But at some point I will probably add some in-check detection, and then I can add this 'anti-chasing' heuristic too.